Morphine and other opioids are extremely effective therapeutic drugs for treating pain, but their antinociceptive effects are limited by detrimental side effects with continued administration, including opioid-induced hyperalgesia, tolerance and dependence. Currently, over $8.6 billion a year are spent in the U.S. for health, work, and legal issues associated with the use and abuse of prescription opioids. The mechanisms underlying these side effects and opioid-induced synaptic plasticity have been difficult to elucidate, primarily because we have a rudimentary understanding of how endogenous pain control systems are activated and regulated by the brain. It is known that presynaptic mu opioid receptors (MOPrs) on GABAergic terminals in the periaqueductal gray area (PAG) activate the descending antinociceptive pathway to the spinal cord. However, MOPrs are also abundantly expressed in the soma and dendrites of PAG neurons and the role of these postsynaptic receptors is not known. We hypothesize that the postsynaptic MOPrs are responsible for the adaptations in opioid tolerance. The proposed studies will test this hypothesis with selective knock-down of the postsynaptic MOPrs using in vivo administration of siRNA constructs. Specific Aim #1 will test whether knock-down of the postsynaptic MOPrs attenuates morphine-induced antinociception. Specific Aim #2 will determine if antinociception can be rescued to normal levels in morphine tolerant rats that have received the siRNA pretreatment. We expect that these studies will confirm that postsynaptic MOPrs regulate the gain of the descending pain modulatory circuit and are appropriate targets for analgesic therapies that do not induce detrimental side-effects such as hyperalgesia, tolerance and dependence with long-term administration. The studies will also lay the groundwork for an R01 application focused on understanding the molecular mechanisms of postsynaptic MOPr regulation of the endogenous descending pain modulatory circuit.